The present invention relates to the general group of doubly salient reluctance machines (DSRMs), including switched reluctance machines (SRMs), also known as variable reluctance machines, stepping motors and hybrid stepping motors producing linear or rotary motion.
Doubly salient reluctance motors have received increasing attention over the past few years, with a large number of publications reviewing their relevant merits with regard to other machine types. The DSRM has been shown to produce a high specific output, despite rather poor utilisation of both the electrical and magnetic circuits, because of the introduction of a magnetic gearing ratio, which arises from the doubly salient nature of the geometry. The magnetic circuit of the machine is poorly utilised because each stator tooth can only be excited to produce positive torque for half of each rotation cycle.
It is to be understood that the term `saliency`, when applied to reluctance machines, implies magnetic saliency, which may or may not involve actual physical saliency.
A doubly salient reluctance machine has a stator and a rotor, both of which exhibit saliency. Magnetic saliency is used here as generally understood in the art, that is, a component of a reluctance machine (either its stator or rotor) is said to be salient if, in operation, changes in the reluctance of the magnetic circuit of the machine occur due to the construction of that component as the relative position of the rotor and the stator changes during operation of the machine.
For example a conventional switched reluctance stepping motor is doubly salient since when a winding in energised and the rotor rotates towards a new position, the main portion of the cross-sectional areas of the active magnetic path in the both rotor and the stator increase and the reluctance of the magnetic circuit decreases due to the construction of both rotor and stator. In operation, energising different windings selects different active magnetic circuits but the topography of a selected circuit varies as the rotor rotates.
A fuller description of switched reluctance motors and their principles and applications can be found in the IEEE Industry Applications Society Tutorial Course Publication "Switched Reluctance Drives" by J. M. Stephenson, S. R. MacMinn and J. R. Hendershot, Jr., as presented on Oct. 12, 1990 at the IEEE IAS Conference In Seattle, Wash. The book "Stepping Motors: a guide to modern theory and practice" by P. P. Acarnley, published by Peter Peregrinus Ltd. on behalf of the Institution of Electrical Engineers, provides an equally useful publication on stepping motors in general.
A related machine which Is not a stepping motor is the synchronous reluctance motor. Such a motor has saliency on the rotor only, the stator being similar to that of an induction motor. A device of this type is disclosed in U.S. Pat. No. 5010267, which describes a variable speed synchronous reluctance machine with a multiphase stator and a rotor divided into segments which constitute flux guides. This machine has a salient rotor, according to the definition of saliency as given above, but the stator has semi-closed slots and is not salient. The topography of the active magnetic path is determined by the flux guides and as the rotor rotates the reluctance of this path changes due to the construction of the rotor only. The stator of the machine of U.S. Pat. No. 5010267 is fully pitched, a fairly common winding arrangement for such machines. Further mention of the significance of fully pitched windings will be made later on in this specification. The specific design of this machine is intended to reduce any effect of mutual inductance between phases as much as possible, as it is recognised that in a machine of this sort mutual inductance will not produce torque which will add to that resulting from the changing self-inductance of each phase.
A further type of related machine is the hybrid stepping motor. Essentially, a permanent magnet provides a component of the magnetic flux in this machine, with currents in at least one stator winding directing the flux along alternative paths. The interaction of the two magnetic fields, one from the rotor magnet and one from the stator windings, produces the torque on the rotor. The arrangement of stator poles and rotor teeth and the selected excitation sequence determine the motion of the rotor. An introduction to and overview of these machines is given in the above-mentioned book by P. P. Acarnley on pages 9 to 11.
Like the switched reluctance motor, this type of machine is also a DSRM. Once again, the stator poles can only be excited to produce torque for half of each rotation cycle, so the machine cannot be utilised to great efficiency.
Another type of related machine is the so-called Vernier reluctance motor,described in the Proceedings of the IEE, Volume 121, No. 9, September 1974 "Vernier Reluctance Motor" by K. C. MuKherji and A. Tustin. This machine has three phase distributed windings, arranged to produce torque due to changing self inductance. Each phase can contribute to positive torque production for a maximum of one half of each cycle.
Mention has already been made of fully pitched windings with relation to the synchronous reluctance motor. The `pole pitch` of a reluctance machine is defined as the peripheral distance between corresponding points on two consecutive simultaneously excited poles of opposite sign, whereas the `coil pitch` is defined as the distance between the two active conductors, or coil sides, of a coil. A fully pitched winding is one in which the ratio of the coil pitch to the pole pitch is 100%, in other words, the two are equal.
Fully pitched windings may be `concentrated` or `distributed`. In the former, the peripheral distance between each coil side of a coil is equal to the pole pitch, and there will generally be one winding slot per phase per magnetic pole. In the latter, each winding is split into a number of regions on each coil side and the peripheral distance between some of these opposed regions will not be the same as the pole pitch.
A salient stator in reluctance machines commonly carries a number of evenly spaced projecting regions, or stator poles, between which the coils are wound in slots. Furthermore, each stator pole may feature a number of projecting teeth to act as flux guides at its extremity. The rotor itself may feature radially projecting portions which in operation define poles and have the effect of making the rotor `salient`. Alternatively, as in some synchronous reluctance machines, the rotor poles may not be readily apparent to the eye. The rotor may have a plurality of salient teeth around its periphery to act as flux guides. How the poles and any teeth of the stator and rotor are arranged depends of course on the precise type and design of machine.